Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session E6: Low Reynolds Number Swimming I |
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Chair: Roman Stoker, Massachusetts Institute of Technology Room: 309 |
Sunday, November 20, 2011 4:40PM - 4:53PM |
E6.00001: Time-dependent flow fields around the spherical colonial alga \textit{Volvox carteri} Douglas Brumley, Marco Polin, Constant Morez, Raymond Goldstein, Timothy Pedley \textit{Volvox carteri} is a spherical colonial alga, consisting of thousands of biflagellate cells. The somatic cells embedded on the surface of the colony beat their flagella approximately towards the south pole, producing a net fluid motion. Using high-speed imaging and particle image velocimetry (PIV) we have been able to accurately analyse the time-dependent flow fields around such colonies. The somatic cells on the colony surface may beat their flagella in a perfectly synchronised fashion, or may exhibit behaviour in which the coordination wanders periodically between forward and backward propagating metachronal waves. We analyse the dependence of this synchronisation on fundamental parameters in the system such as colony radius, characterise the speed and wavelength of metachronal waves propagating on the surface, and investigate the extent to which hydrodynamic interactions are responsible for the exhibited behaviour. The time-averaged flow fields agree with previous experiments involving freely swimming colonies (Phys. Rev. Lett. 105:168101, 2010) and Blake's squirmer model (J. Fluid Mech. 46, 199-208, 1971b). [Preview Abstract] |
Sunday, November 20, 2011 4:53PM - 5:06PM |
E6.00002: Particle Image Velocimetry Around Swimming Paramecia Matthew Giarra, Saikat Jana, Sunghwan Jung, Pavlos Vlachos Microorganisms like paramecia propel themselves by synchronously beating thousands of cilia that cover their bodies. Using micro-particle image velocimetry ($\mu $PIV), we quantitatively measured velocity fields created by the movement of \textit{Paramecium multimicronucleatum} through a thin ($\sim $100 $\mu $m) film of water. These velocity fields exhibited different features during different swimming maneuvers, which we qualitatively categorized as straight forward, turning, or backward motion. We present the velocity fields measured around organisms during each type of motion, as well as calculated path lines and fields of vorticity. For paramecia swimming along a straight path, we observed dipole-like flow structures that are characteristic of a prolate-spheroid translating axially in a quiescent fluid. Turning and backward-swimming organisms showed qualitatively different patterns of vortices around their bodies. Finally, we offer hypotheses about the roles of these different flow patterns in the organism's ability to maneuver. [Preview Abstract] |
Sunday, November 20, 2011 5:06PM - 5:19PM |
E6.00003: High Speed Tomographic PIV Measurements of Copepod Sensory Cues Donald R. Webster, David Murphy, Jeannette Yen A steady siphon flow is commonly used to mimic the aquatic suction feeding of piscine predators in studies of zooplankton sensory ecology. The sensory and escape behavior of copepods, with their long, highly enervated, setae-bearing antennules, has been investigated using this prescribed flow field, modeled analytically as a point sink. The position of the animal when it escapes provides a threshold for the species-specific strain rate value (as low as 0.4 s$^{-1})$ that evokes this ecologically-important behavior. Understanding the actual mechanics of copepod sensing, however, requires more than just correlation analysis based on position in the strain rate field. Knowledge of the setae-bending flow field along the length of the antennules during the time leading up to the escape jump is needed to fully understand the sensory mechanism. Measurements of this type have not been practical using traditional planar PIV techniques. We present time-resolved tomographic PIV measurements of the flow around the sensory appendages of \textit{Acartia} spp. copepods responding to siphon flow and provide insight into copepod behavior in response to fluid mechanical stimuli. [Preview Abstract] |
Sunday, November 20, 2011 5:19PM - 5:32PM |
E6.00004: A rigorous proof of the scallop theorem and finite mass effects of a microswimmer Kenta Ishimoto, Michio Yamada We reconsider fluid dynamics of a self-propulsive swimmer in Stokes flow. With an exact definition of deformation of a swimmer, a proof is given to the scallop theorem including the body rotation of the swimmer. Introducing a virtual swimmer, which has the same shape as the real swimmer but has no ambient fluid, we give an exact definition of the surface deformation of the generally translating and rotating swimmer, and then prove the scallop theorem rigorously for the massless swimmer. We also discuss the breakdown of the scallop theorem due to a finite mass (finite Stokes number) of the swimmer by using a perturbation expansion method and it is found that the breakdown generally occurs at the first order of Stokes number. In addition, employing the Purcell's ``scallop'' model, we show that the theorem holds up to a higher order of the Stokes number, if the swimmer's stroke has some symmetry. \\[4pt] [1] K. Ishimoto and M. Yamada, ``A rigorous proof of the scallop theorem and a finite mass effect of a microswimmer,'' arXiv:1107.5938v1 [physics.flu-dyn]. [Preview Abstract] |
Sunday, November 20, 2011 5:32PM - 5:45PM |
E6.00005: Performance of flexible low-Re swimmers in Newtonian and viscoelastic liquids J. Espinosa, R. Zenit, E. Lauga We show experimental results of ``flexible tail'' swimmers in elastic fluids. A magnetic microswimmer powered by a frequency-controlled homogeneous magnetic field was built. Experiments were performed in a reference viscous Newtonian fluid and a glucose-based Boger fluid of the same shear viscosity. High definition video of the swimmer traveling along a channel was taken to measure its average swimming speed. We found that locomotion is enhanced in elastic fluids for most conditions. To further investigate the swimming performance, the flow field around the swimmer was visualized with a PIV (Particle Image Velocimetry) technique. The differences between Newtonian and Boger fluid will be presented and discussed. [Preview Abstract] |
Sunday, November 20, 2011 5:45PM - 5:58PM |
E6.00006: Locomotion of helical swimmers in Newtonian and complex fluids F.A. Godinez, R. Zenit, E. Lauga A biomimetic swimming device was built using a magnetic head and a rigid helical tail to investigate experimentally the effects of viscoelasticity on creeping flow locomotion. The millimeter-size swimmer is controlled and actuated wirelessly using a rotating magnetic field. Two fluids with similar viscosities were used for experiments, one Newtonian and one viscoelastic Boger fluid. Preliminary results from both fluids show a linear relationship between the frequency of the applied field and the translational velocity. The objective of this investigation is to determine if the viscoelasticity of the fluids enhances or not the swimming performance for this device. [Preview Abstract] |
Sunday, November 20, 2011 5:58PM - 6:11PM |
E6.00007: On growth and flow: bacterial biofilms in porous media William Durham, Alberto Leombruni, Olivier Tranzer, Roman Stocker Bacterial biofilms often occur in porous media, where they play pivotal roles in medicine, industry and the environment. Though flow is ubiquitous in porous media, its effects on biofilm growth have been largely ignored. Using patterned microfluidic devices that simulate unconsolidated soil, we find that the structure of \textit{Escherichia coli} biofilms undergoes a self-organization mediated by the interaction of growth and flow. Intriguingly, we find that biofilm productivity peaks at intermediate flow rates, when the biofilm is irrigated by a minimum number of preferential flow channels. At larger and smaller flow rates, fluid flows more uniformly through the matrix, but productivity drops due to removal by shear and reduced nutrient transport, respectively. These dynamics are correctly predicted by a simple network model. The observed tradeoff between growth and flow may have important consequences on biofilm-mediated processes such as biochemical cycling, antibiotic resistance and water filtration. [Preview Abstract] |
Sunday, November 20, 2011 6:11PM - 6:24PM |
E6.00008: Maneuverability at low Reynolds number Lisa Burton, Ross Hatton, Howie Choset, A.E. Hosoi Speed and efficiency are common and often adequate measures to compare swimming systems. However, these metrics do not take into account how well a system can reconfigure, change direction, or move in a confined environment. Inspired by manipulability, a concept used to analyze robotic arms, we propose new metrics to consistently quantify the maneuverability of a system. We discuss a general definition of maneuverability and apply it to low Reynolds number swimming. Additionally, we identify the library of motions to maximize an artificial swimmer's maneuverability and explore the effect of morphology and kinematics on maneuverability. [Preview Abstract] |
Sunday, November 20, 2011 6:24PM - 6:37PM |
E6.00009: Microswimmers in Complex Environments with Heterogeneous Microstructure YunKyong Hyon, Henry Fu We will discuss the swimming of microorganisms in complex and heterogeneous environments. Microswimmers in biological complex fluids, for instance, bacteria and sperm, are often greatly influenced by heterogeneous medium microstructure with length scales comparable to themselves. We characterize the interaction between the microswimmer and the medium microstructure using the model Golestanian three-sphere swimmer, treating the hydrodynamic interaction with the microstructure through the Oseen tensor. In this investigation, the microstructure of the heterogeneous environment is modeled by fixed spheres representing obstacles, or chains consisting of spheres connected with elastic springs. We find that the swimming speed of the swimmer depends on the force and deformation exerted on micro-structure. Furthermore, we find that while short freely suspended chains and short chains anchored at their ends interact with swimmer quite differently, long enough chains interact similarly, that is, a long mobile chain acts like a anchored chain. We discuss the implications for swimmer interactions with polymer solutions and compliant networks. [Preview Abstract] |
Sunday, November 20, 2011 6:37PM - 6:50PM |
E6.00010: Swimming speed of an oscillating sheet in Newtonian and viscoelastic fluids Moumita Dasgupta, Michael Berhanu, Arshad Kudrolli, Bin Liu, Kenneth Breuer, Thomas Powers We discuss a mechanical experimental model of a flexible sheet swimming with a prescribed wave pattern through a fluid. We are motivated by a need for a fundamental understanding of microorganism locomotion through non-Newtonian fluids. To simplify the problem, we suspend a tall flexible cylindrical sheet concentric within a cylindrical tank filled with the fluid. Torque free boundary conditions are imposed by supporting the flexible sheet and the tank with friction-free ball-bearings. A traveling wave is imposed on the sheet with a pair of rollers in the azimuthal direction. We first show that the swimming speed is linear with respect to the phase velocity of the traveling wave for a viscous Newtonian fluid. Then we show that the system is essentially two dimensional as the results do not depend on the height of fluid in the tank. We measure swimming speed in Polyox-water mixtures and Sodium CMC solutions as a function of wave speed. We again demonstrate linear response in the swimming speeds, which also decrease in these viscoelastic fluids relative to the Newtonian case as wave speed increases. Decrease in swimming speed is observed with increase in viscoelasticity of the fluids. We then discuss the dependence of swimming speed on the Deborah number of the fluids. [Preview Abstract] |
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